One of the victims of the triple killing was a boxer and friend of Tamerlan Tsarnaev (above). Photograph: Reuters

Boston Marathon bombing suspect Tamerlan Tsarnaev was named as a participant in an earlier triple homicide by a man who was shot to death while being questioned by authorities, according to a filing by federal prosecutors in the case against his brother, surviving suspect Dzhokhar Tsarnaev.

According to the filing on Monday, Ibragim Todashev told investigators Tamerlan Tsarnaev, who was shot dead by police shortly after the bombing, took part in a triple murder in Waltham on 11 September 2011.

In that case, three men were found in an apartment with their throats cut and their bodies reportedly covered with marijuana. One of the victims was a boxer and friend of Tamerlan Tsarnaev.

Todashev, a 27-year-old martial arts fighter, was fatally shot at his Orlando home during a meeting with an FBI agent and two Massachusetts state troopers in May, according to authorities. He had turned violent while being questioned, they said.

The filing is the prosecutors' attempt to block Dzhokhar Tsarnaev from getting certain information from authorities, including investigative documents associated with the Waltham killings.

"The government has already disclosed to Tsarnaev that, according to Todashev, Tamerlan Tsarnaev participated in the Waltham triple homicide," prosecutors wrote.

According to prosecutors, the investigation into the 2011 murders is reason not to allow Dzhokhar Tsarnaev access to documents.

"Any benefit to Tsarnaev of knowing more about the precise 'nature and extent' of his brother's involvement does not outweigh the potential harm of exposing details of an ongoing investigation into an extremely serious crime, especially at this stage of the proceeding," prosecutors wrote.

Prosecutors also said Dzhokhar Tsarnaev was not entitled to the information because his brother's criminal history would be relevant only at a possible future sentencing hearing, if at all.

A phone message left for a spokeswoman for the US attorney's office was not immediately returned on Tuesday night. A message left for Dzhokhar Tsarnaev's federal public defender was also not immediately returned.

Authorities allege that Dzhokhar Tsarnaev, 20, and 26-year-old Tamerlan Tsarnaev, ethnic Chechens from Russia, planned and carried out the twin bombings near the finishing line of the marathon on 15 April. Three people were killed and more than 260 injured in the attack.

Dzhokhar Tsarnaev faces 30 federal charges, including using a weapon of mass destruction and 16 other charges that carry the possibility of the death penalty.

Tamerlan Tsarnaev died in a gun battle with police as authorities closed in on the brothers several days after the bombings.